Method for controlling an artificial orthotic or prosthetic knee joint

ABSTRACT

The invention relates to a method for controlling an artificial orthotic or prosthetic knee joint, on which a lower leg component is arranged and which is assigned a resistance device having at least one actuator, by means of which the bending resistance is modified depending on sensor data that is determined during use of the orthotic or prosthetic knee joint by means of a sensor, wherein the absolute angle of the lower leg component is determined exclusively by means of at least one inertial sensor, the angle determined is compared with at least one threshold value, and the bending resistance is modified when the threshold value is reached.

The invention relates to a method for controlling an artificial orthotic or prosthetic knee joint, on which a lower leg component is arranged and which is provided with a resistance device having at least one actuator, by means of which the bending resistance is modified as a function of sensor data which are determined by means of at least one sensor during use of the orthotic or prosthetic knee joint.

Prosthetic or orthotic knee joints replace or support the function of a natural knee joint. In order to achieve a maximally optimal functionality of the artificial knee joint, there are a multiplicity of designs on the market, which influence the behavior of the knee joints during the standing phase and the swinging phase. Mechatronic knee joints are known, in which the movement situations are detected by means of a plurality of different sensors and, on the basis of the sensor data, a resistance device, by means of which the bending resistance or the extension resistance is varied, is controlled. One basic problem is that the great variety of the possible movement situations can be encompassed only with difficulty in simple rules. In order to control actuators and brakes, therefore, so-called state machines are used, which are highly complex and represent many different activities. Disadvantages with this are the long development time and the use of elaborate components.

EP 1 237 513 B1 relates to a supporting device which replaces the existence or function of a limb and consists of at least two parts, connected to one another by an artificial joint, and a control device. The supporting device comprises a sensor, which detects an inclination angle relative to a line of gravity of a part connected to the joint and is coupled to the control device. The control device is arranged in such a way that it influences the joint on the basis of inclination angle data communicated by the sensor. In one configuration, the inclination angle sensor is arranged as a prosthetic knee joint on a thigh tube; in order to enhance the data situation, a second sensor may be arranged on the lower leg.

DE 10 2008 027 639 A1 relates to an orthotic joint for supporting an anatomical knee joint, having an upper joint part and a lower joint part which are connected to one another in an articulated fashion. A locking element for automatically unblocking and blocking the orthotic joint in an arbitrary position is provided, as is an actuation element for the locking element and a sensor means for detecting relevant information for the unblocking and blocking. An evaluation unit for evaluating the information acquired, and for forwarding this information to a control and/or regulating unit for the actuation element, is likewise provided. The sensor means comprises at least two sensors from the group: inclination sensors, rotation angle sensors, acceleration sensors or gyroscopes, for acquiring information describing the movement state and/or resting state of a person. Two sensors of one type or one sensor each of different types may be selected. All the sensors are arranged downward of the anatomical joint, in particular knee joint.

It is an object of the present invention to provide a method for controlling an artificial orthotic or prosthetic knee joint, with which a reliable and comfortable walking behavior of the prosthesis or orthesis can be achieved in a simple and economical way.

According to the invention, this object is achieved by a method having the features of the main claim. Advantageous configurations and refinements of the invention are disclosed in the dependent claims, the description and the figures.

According to the method for controlling an artificial orthotic or prosthetic knee joint, on which a lower leg component is arranged and which is provided with a resistance device having at least one actuator, by means of which the bending resistance is modified as a function of sensor data which are determined by means of at least one sensor during use of the orthotic or prosthetic knee joint, the absolute angle of the lower leg component is determined exclusively by means of at least one inertial sensor, the angle determined is compared with at least one threshold value, and the bending resistance is modified, in particular reduced, when the threshold value is reached, in particular exceeded. In this way, sensor data which detect various measurement quantities, for example a torque, a force and an angle, which then need to be processed relatively elaborately, for a control algorithm can be obviated. The restriction to the use of absolute angles, which are measured by means of one or more inertial sensors, the absolute angle of the lower leg component being determined, is a simple and at the same time surprisingly reliable way in which control of a resistance change in a resistance or damping device of an artificial orthotic or prosthetic knee joint can be produced. In this case, furthermore, a plurality of threshold values are established, which trigger adaptation of the bending resistance when reached or exceeded or fallen below. In this way, it is possible to achieve a damping curve which is adapted to the patient and allows a smooth gait.

According to one refinement of the invention, an angular velocity of the lower leg component is calculated from the sensor data of the inertial sensors, and the bending resistance is reduced only when the angular velocity is not equal to zero, i.e. the lower leg component is moved in the extension or flexion direction. This ensures that a resistance change is carried out only during walking, as a function of the absolute angle of the lower leg component. The combination of an absolute angle with the angular velocity for the detection of a swinging phase, and therefore establishment of the time when a bending resistance is reduced from standing phase damping to swinging phase damping, has been found to be reliable even in the case of slow walking speeds. It is also possible to reliably determine other phases of the movement during walking by the combination of an absolute angle and angular velocity, so that the bending resistance or the extension resistance can be varied in a scope such that a smooth gait is obtained and the patient receives reliable and effective support.

According to one refinement of the invention, the absolute angle is determined exclusively by means of one or more inertial sensors, which is or are fastened on the lower leg component or on an orthotic or prosthetic component fastened distally thereon. The distally arranged orthotic or prosthetic components are, in particular, prosthetic feet or braces or arch supports for a natural foot in the case of an orthesis. The sensor or the sensors are preferably fastened medially and/or laterally on the respective lower leg component, in order to determine the absolute angle of the lower leg component, i.e. its position with respect to a fixed reference quantity, in particular the vertical.

The absolute angle may be determined by means of 2D or 3D magnetic field sensors, 2D or 3D acceleration sensors and/or 1D, 2D or 3D gyroscopes. When a plurality of sensors are used, the sensor data of a plurality of inertial sensors may be merged together in order to be able to reliably establish the actual orientation of the lower leg component on the basis of a plurality of different sensor signals. Measurement inaccuracies or perturbations in the case of one sensor can then be compensated for by the other sensor or sensors.

The threshold value of the absolute angle of the lower leg component may be adjusted to the value which the lower leg component adopts at the end of the standing phase, so that particular physical properties of the patient can be taken into account by the individual adjustment. Likewise, a preferred movement pattern may be taken into account by adaptation of the threshold value of the absolute angle individually to the gait of the patient by the orthopedic technician responsible. The end of the standing phase is to be understood as meaning the instant within a walking cycle at which the front of the foot just still touches the ground during the rolling of the foot, immediately before the front of the foot loses contact with the ground and is lifted. The knee joint usually then bends further, so that flexion in the knee joint increases the distance of the foot from the ground.

The bending resistance may be switched between two fixed values so that there is a low bending resistance during the swinging phase and a high bending resistance during the standing phase, or in an emergency mode. Such an emergency situation may be activated in the event of falling or stumbling. An increased bending resistance may then be provided in order to prevent unbraked folding of the knee. The configuration of the actuator is advantageously selected in such a way that triggering by the actuator in order to reduce the damping cannot take place when there is an applied flexion moment. This may, for example, be achieved by dimensioning the power of the actuator to be less than the power which is necessary in order to overcome the counterpressure of a prosthetic joint loaded with a flexion moment. In this way, an actuation lock of the actuator is achieved by virtue of the technical power configuration or mechanical configuration, so that additional controls or sensors are not necessary. If the prosthesis user is in a position with a folded knee which is loaded, then, owing to the selected low power of the actuator, adjustment against the internal pressure of the system cannot take place, so that opening of the damping and spontaneous reduction of the damping can be avoided in potentially hazardous situations, even if the predetermined other parameters for varying the flexion damping are fulfilled.

According to one refinement of the invention, bending resistance is modified, in particular increased, when the angular velocity has reached a zero point and reversal of the movement direction of the lower leg component is determined. If the bending resistance is increased at the end of swinging phase flexion, then increased security against unintended folding in the event of collision with an obstacle is provided. If the patient stumbles, increasing the bending resistance makes it possible for the patient to be supported by the leg provided with the orthesis or prosthesis, without the knee joint folding. If reversal of the movement direction is determined at the end of the swinging phase, the bending resistance may be reduced in order to permit the initial standing phase flexion. Reversal of the movement direction at the end of the swinging phase may be determined by the angular velocity reaching a zero point and the absolute angle being reduced.

The angular acceleration of the lower leg component may also be determined from the inertial sensor data, provision being made for the bending resistance to be increased, or not reduced, when a threshold value is exceeded. This is used in order to detect special situations, for example when stumbling. The angular acceleration is preferably determined when walking forward, so that the most frequent walking situations can be recorded and taken into account. Exceeding of a limit value by the angular acceleration can indicate that the smooth walking movement is interrupted, so that a damping value other than that of the swinging phase damping seems expedient. In general, the flexion damping is then to be increased or left at a high value.

An exemplary embodiment of the invention will be explained in more detail below with the aid of the figures, in which:

FIG. 1—shows a schematic representation of a prosthetic device;

FIG. 2—shows a flow chart of the control; and

FIG. 3—shows a lower leg angle diagram.

FIG. 1 provides a schematic representation of a prosthetic leg with a prosthesis shaft 1 for receiving an upper leg stump and for fixing the prosthetic leg to a patient. Arranged at the distal end of the prosthesis shaft 1, there is a prosthetic knee joint 2 which is equipped with a resistance device 3, for example in the form of a hydraulic damper or a wrap spring brake. At the distal end of the prosthetic knee joint 2, a lower leg tube 4 and a prosthetic foot 5 are provided as further distal components. The functional elements: prosthesis shaft 1, prosthetic knee joint 2, lower leg tube 4 and prosthetic foot 5 are enclosed by a cosmetic covering 6 in order to impart a natural overall impression as far as possible.

In the exemplary embodiment represented, an inertial sensor 7 as an angle pickup is arranged on the lower leg component, which consists of the distal part of the prosthetic knee joint 2, the lower leg tube 4 and the prosthetic foot 5. The inertial sensor 7 may be formed as a magnetic field sensor, acceleration sensor or gyroscope. It is also possible for a plurality of inertial sensors 7 to be arranged on the lower leg component, for example in addition to the fitting at the distal part of the prosthetic knee joint 2 on the lower leg tube 4 or the prosthetic foot 5. The acceleration sensors and magnetic field sensors may be formed as 2D or 3D sensors, and in order to determine gyroscope data a gyroscope may be formed as a 1D, 2D or 3D gyroscope. A plurality of inertial sensors of the same type may be arranged on the lower leg component, likewise inertial sensors 7 of different types, for example an acceleration sensor and a gyroscope, may be fixed on the lower leg component.

The inertial sensor 7, which is formed as an angle sensor, determines the angle value of the lower leg component relative to a center of mass line 8, which extends through a center of gravity 9. The center of gravity 9 corresponds to the center of mass of the body of the patient, and the angle α is determined between the center of mass line 8 and the longitudinal extent of the lower leg component through the center of mass 9 of the body in the extended position of the prosthetic knee joint 2 at the end of the standing phase. The orientation of the lower leg component in FIG. 1 is defined by the connecting straight line 10 along the longitudinal extent of the lower leg tube 4 through the tilt axis of the prosthetic knee joint 2. In the position represented, the prosthetic knee joint 2 is in the extension position at the end of the standing phase. The existing angle α of the lower leg component relative to the center of mass line 8 is stored as a threshold value. At this value, there is extension of the prosthetic knee joint 2 and of the hip joint, and therefore a step rear position of the leg, and it is safe for the user to initiate the swinging phase. If a rolling process of the prosthetic foot 5 in the anterior direction is simultaneously detectable, which is manifested by an angular velocity α′>0 and the angle increase, there is likewise a step in the anterior direction. On the basis of these data, an actuator 31 of the resistance device 3 of the prosthetic knee joint 2 is activated in such a way that existing standing phase damping is reduced and is kept low until the angular velocity α′ has reached a zero point in the central swinging phase. An angle range for the position of the lower leg component at the end of the swinging phase of a normal step may be stored on the basis of empirical values. If a full step is not executed, i.e. the angle range is not reached, or a non-smooth variation in the angular velocity α′ is established, the resistance device 3 is influenced by means of the actuator 31 in such a way that there is an increased bending resistance. If the interrupted step is continued, a reduction of the flexion resistance may be carried out again in order to end the step.

FIG. 2 represents the functionality of the control as a diagram. After the start of the control, the respective sensor value is determined in a first step 20. A plurality of sensor values 7₁, 7₂, 7₃ may be measured by means of the inertial sensors 7. Besides the possibility of determining three sensor values 7₁, 7₂, 7₃ of the lower leg component, for example by using three different inertial angle sensors 7 such as a magnetic field sensor, an acceleration sensor and a gyroscope, it is also possible to determine a plurality of sensor values by using a plurality of inertial sensors 7 of the same type. In principle, it is also possible to detect the sensor value with only one inertial sensor 7.

The data detected by the inertial sensors 7 are merged in a further working step 21, in order to compensate for inaccuracies and have a data situation which is as complete as possible for the calculation of the absolute angle α. If only one inertial sensor 7 is provided, the data do not need to be merged.

In a subsequent evaluation step 22, the sensor data 7₁, 7₂, 7₃ of the angles α of the lower leg component with respect to the line of gravity 8 are calculated. It is likewise possible to calculate the angular velocity α′ of the lower leg component in parallel therewith in a further working step 23.

The angle α, calculated for example by a Kalman filter, with respect to the line of gravity 8, is then compared in a further step 24 with a threshold value X which was established beforehand. As soon as the absolute angle α is greater than the preset threshold value X, in a control situation in which the angular velocity α′ is not taken into account, the actuator 31 may be activated in a further step 26, in such a way that the damping device 3 adopts a reduced resistance for initiation of the swinging phase. If the threshold value is not reached, in the alternative step 27 the actuator 31 is not correspondingly actuated and the resistance of the resistance device 3 remains unchanged.

If the angular velocity α′ is also calculated together with the angle α in step 23, and the angular velocity α′ is greater than zero, then a walking movement is detected in step 28. In a combining step 25, the angle α and the angular velocity α′ are coupled with one another, and if both threshold values are present or are exceeded, the actuator is activated according to step 26, while if one of the two threshold values for the angle α or the angular velocity α′ is absent, the actuator 31 is not activated according to step 27.

FIG. 3 represents a diagram of the lower leg angle a as a function of time. The individual positions during a stepping cycle are represented above the diagram. The step begins with planting of the heel, the so-called heel strike, so that the lower leg angle α is initially slightly increased. In the course of continued stepping, the prosthetic foot is fully planted and after about 0.4 second the lower leg, or the lower leg component, reaches the vertical, so that the lower limb angle α is 0°. As a result of movement further forward, the lower leg, or the lower leg component, is tilted further so that the lower leg α is increased in magnitude. The end of the standing phase is reached after about 0.8 second, and the front of the prosthetic foot leaves the ground so that swinging phase flexion occurs. At the end of the swinging phase, in the case of normal walking, the reversal point is reached at a lower leg angle of 45°. This is the case at about 1.3 seconds. A movement reversal subsequently takes place, the lower leg angle α is reduced in magnitude and approaches the vertical, until at about 1.7 seconds it reaches the vertical again and is then tilted further relative to the vertical, but this time in the positive angle range, by the foot being extended forward. Over the profile of the lower leg angle α, a plurality of threshold values may be established for this lower leg angle α. If these threshold values are reached or exceeded, or fallen below, depending on the direction in which the threshold value is considered, the variation of the bending resistance may be carried out; in particular, after a maximum bending angle is reached, the bending resistance may be increased in order to provide security against unintended and unbraked folding of the knee joint in the event of stumbling. Simple and effective control of the damping of the resistance device can be carried out by means of the setting of the lower leg relative to the vertical.

It has been found that the simple algorithm described above can be used well in order to switch to and fro between standing phase damping and swinging phase damping. The control is particularly reliable when the angle value α is greater than a previously stored threshold value, which is adjusted by operating a control knob on the prosthetic device when the patient is in backward movement and the angular velocity α′ is greater than zero. If both conditions are satisfied, the resistance device 3 may be switched over from high standing phase damping to lower swinging phase flexion damping by switching a hydraulic valve or by releasing a lock of a brake device, for example.

The use exclusively of inertial sensors reduces the costs, since the torque sensors based on strain gauges for alternative methods are very expensive. Furthermore, inertial angle sensors are free from wear.

Besides the first derivative of the angle signal for determining the angular velocity, it is also possible to use the second derivative of the angle signal, in order to determine the angular acceleration. The acceleration signal may be used to detect falling, the angular acceleration exceeding a fixed limit value indicating that the smooth walking movement is interrupted and a different damping value would be more expedient, usually increasing the flexion resistance.

Furthermore, by observing the angle profiles, it is also possible to deduce the walking speed; in addition, not only may the resistance behavior, and therefore the damping behavior during the swinging phase, be switched to and fro between two values in a binary fashion, but the switching may be adjusted in very small time intervals for discrete angle values or for any angle value, or any sampling of the angle value. For example, hydraulic valves may be adjusted stepwise or brake devices, which may likewise be used in resistance devices, may be adjusted in an adapted fashion in order to produce a smooth gait.

Furthermore, on the basis of the angle profiles, the resistance device may be adjusted in such a way as to allow standing phase flexion in the event of a heel strike with an extended joint, by reducing the flexion resistance. After this has happened, a progressive variation of the damping may also be provided, so that, after reduction of standing phase flexion after the heel strike, strongly increasing damping occurs, which allows bending up to a particular angle value after the heel strike; in addition, a further increase in the angle is prevented by adjustment of the resistance device.

The artificial knee joints may be used both as prostheses, as described in the exemplary embodiment, and as ortheses. The resistance devices may be configured as simple locks, complex hydraulics or wrap spring brakes. By taking the angular velocity α into account, it is also possible to use drives in order additionally to permit folding or extension. For security against undesired folding or reduction of the damping under load, i.e. in the case of an applied flexion moment, the actuator may be configured in terms of power so that the switching power to be applied when a flexion moment occurs lies above the output power of the actuator, so that reduction of the damping and therefore abrupt folding cannot occur. 

1. A method for controlling an artificial orthotic or prosthetic knee joint, on which a lower leg component is arranged, the method comprising: providing the orthotic or prosthetic knee joint with at least one inertial sensor and a resistance device, the resistance device having at least one actuator; determining an absolute angle of the lower leg component exclusively using sensor data from the at least one inertial sensor; comparing the determined absolute angle with at least one threshold value; modifying a bending resistance with the resistance device when the threshold value is reached.
 2. The method as claimed in claim 1, further comprising: calculating an angular velocity of the lower leg component using the sensor data of the at least one inertial sensor; reducing the bending resistance only when the angular velocity is not equal to zero.
 3. The method as claimed in claim 1, wherein the absolute angle is determined exclusively using one or more inertial sensors, which is or are fastened on the lower leg component or on an orthotic or prosthetic component fastened distally thereon.
 4. The method as claimed in claim 1, wherein the absolute angle is determined using at least one of 2D or 3D magnetic field sensors, 2D or 3D acceleration sensors, and 1D, 2D or 3D gyroscopes.
 5. The method as claimed in claim 1, further comprising merging together the sensor data of a plurality of inertial sensors.
 6. The method as claimed in claim 1, further comprising adjusting the threshold value of the absolute angle of the lower leg component to a value which the lower leg component adopts at an end of the standing phase.
 7. The method as claimed in claim 1, further comprising switching the bending resistance between two fixed values.
 8. The method as claimed in claim 1, further comprising modifying the bending resistance when an angular velocity has reached a zero point and reversal of a movement direction of the lower leg component is determined.
 9. The method as claimed in claim 1, further comprising determining an angular acceleration of the lower leg component, and increasing, or not reducing, the bending resistance when the threshold value is exceeded.
 10. A method for controlling an artificial orthotic or prosthetic knee joint, the method comprising: providing the orthotic or prosthetic knee joint with at least one inertial sensor and a resistance device; determining an absolute angle of a lower leg component mounted to the orthotic or prosthetic knee joint using sensor data only from the at least one inertial sensor; comparing the determined absolute angle with at least one threshold value; modifying a bending resistance in the orthotic or prosthetic knee joint with the resistance device when the threshold value is reached.
 11. The method as claimed in claim 10, further comprising: calculating an angular velocity of the lower leg component using the sensor data of the at least one inertial sensor; reducing the bending resistance only when the angular velocity is not equal to zero.
 12. The method as claimed in claim 10, wherein the at least one inertial sensors is fastened on the lower leg component or on an orthotic or prosthetic component fastened distally thereon.
 13. The method as claimed in claim 10, wherein the absolute angle is determined using at least one of 2D or 3D magnetic field sensors, 2D or 3D acceleration sensors, and 1D, 2D or 3D gyroscopes.
 14. The method as claimed in claim 10, further comprising merging together the sensor data of a plurality of inertial sensors.
 15. The method as claimed in 10, further comprising adjusting the threshold value of the absolute angle of the lower leg component to a value which the lower leg component adopts at an end of the standing phase.
 16. The method as claimed in claim 10, further comprising switching the bending resistance between two fixed values.
 17. The method as claimed in claim 10, further comprising modifying the bending resistance when an angular velocity has reached a zero point and reversal of a movement direction of the lower leg component is determined.
 18. The method as claimed in claim 10, further comprising determining an angular acceleration of the lower leg component, and increasing, or not reducing, the bending resistance when the threshold value is exceeded. 